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Stoichiometry: Introduction to limiting reactants

Added Feb 02, 2016 •

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in this video we're going to talk about limiting reactant which can also be called limiting reagent along the way will also discuss a related topic call excess reactant limiting reactant excess reactant the stuff all has to do with chemical reactions but to explain it from the beginning I actually don't want to start talking about the chemistry instead I want to start talking about some analogies to cook will compare a chemical to a cooking recipe let's say you're having some friends over for barbecue you want to make some cheeseburgers alright here's what you're going to need to make you're going to need a bun you're going to need a slice of cheese and you're going to need a piece of meat since we're comparing chemistry and okay we can write the recipe for this kind of like a chemical equation alright here it is one bun plus one slice of cheese Plus a piece of meat put that all together and it gives us one cheeseburger the ingredients over here that we start with we can think of those kind of like reactants in a chemical reaction and the products over here cheeseburger is what we end up with okay so now we've got our recipe to do some cooking we got to get some ingredients that's rummage around in the refrigerator so we can find so here are our ingredients and you'll notice we have different numbers of each one this happens a lot also happens a lot in chemical reactions you often start with different amounts of each reactant so we're talking about limiting reactant here in a limiting reactant comes into play when we asked the question given these different numbers of ingredients what is the greatest number of cheeseburgers that we can make alright we got four buns here we got nine slices of cheese but if we've only got three pieces of meat we can really only make 3 cheeseburgers right check this out here's a first cheeseburger here's a second one and here's the third one and then we run out of meat and we can't make any more cheeseburgers If This Were chemical reaction we'd say that meat is the limiting reactant okay here's the definition for reactant is the first reactant that used up in a when the limiting reactant is all used up no more product conform and the reaction stops so clearly play some gradients that was used up as we're making the cheeseburgers and as soon as we use it all up no more product inform we can't make any more cheeseburgers and whole process has stop limiting reactant because it limits how much of the product we can make right it doesn't matter if we have box for 100 slices of cheese once we run out of those three pieces of meat we're done we can't make any more cheeseburger so the total number cheeseburgers and we can make is completely dependent on the amount of the meet the limiting reactant that we have so after we run out of the meat we still have some extra ingredients left up here and these are what we call excess reactants  excess reactant is what is left over after the reaction stops because the limiting reactant got all used up okay so we have some excess reactant we got the bun here we have one excess bun and then some more excess reactant we have six piece of cheese that are left over after the meat gets all used up okay so now that we have talked about the concepts behind limiting reactant and excess reactant let's use this information to solve some problems that are little good morning vanced this looks more like a chemical equation but we're still cooking here I'm pretending this is a recipe for bread rolls it would actually be a terrible recipe it's just flour and water I definitely would want to eat these bread rolls but this is going to be great practice for limiting reactant and excess reactant so we're going to use this recipe or this equation the solve the following problem what is the greatest amount of bread rolls that can be made with 6 cups of flour and 3 cups of water If This Were chemical reaction what would be the limiting reactant which reactant is in excess and how much is left over so we want to make the greatest amount of bread rolls with 6 cups of flour and 3 cups of water let's get these ingredients out here here they are figure out the greatest amount of red rolls we can make we first have to identify which of these ingredients is the limiting reactant that's a little bit tricky then it was with a cheeseburger problem we just did we can't only look at these and figure out which is a limiting reactant instead we're going to have to solve this problem like where to start with say the flower and we're going to ask if we want to use the maximum amount of this flower how much water are we going to need do we have enough or is it going to run out and then we're going to switch and we're going to do the opposite we're going to ask if we can use the maximum amount of water we want to use all three cups of this how much flour do we need and do we have enough or is it going to run okay let me show you what I mean what start with a flower can we got six cups here no this recipe is only for three cups so what if you were in the kitchen and you actually wanted this recipe with six cups of flour instead of 3 what would you do you multiply everything by 2 okay so we can multiply this by 2 to make the recipe with 6 cups of flour for using 6 cups of flour how much water do we need what we can multiply this by 2 which will tell us that we need two cups of water to go with a 6 cups of flour and finally how many bread rolls is that going to make what we're just doubling the recipe sounds 5 we're going to end up with 10 okay so that's for trying to use the maximum the six cups of flour what are we getting make a terms of flower if we want to use the maximum amount of water okay we got three cups of water okay the recipe is for one cup of water so it's kind of like we got a triple the recipe or multiply 1 by 3 how about the flour how much flour are we going to need for 3 cups of water while we triple this so we have 3 x 3 we're going to need 9 cups of flour how many bread rolls are we going to get Triple the recipe 5 * 3 15 Bros alright so here's all of our information what's the greatest amount of bread rolls that we can make one of these situations isn't going to work because we're going to run out of one of the ingredients okay let's take a look at this if we start with six cups of flour we need two cups of water do we have that yeah we do we have three cups we actually have more than enough so this would work we could make ten bread rolls what if we start with a three cups of water then we need 9 cups of flour but that's not going to work because we only have 6 cups of flour so we can make 15 bread rolls we don't have enough flour the flower is going to be the first ingredient for the first reactant to run out so that means that flower he is the limiting reactant it's going to limit the amount of bread rolls that we can make so flower here is the limiting reactant now we can so show this visually okay I imagine what kind of cooking this recipe okay 3 cups of flour and one cup of water get combined okay so here are three cups of flour one cup of water we take them put them in the oven or whatever we get 5 bread rolls 1 2 3 4 5 okay let's do that again another 3 cups of flour 1 cup of water combine them and we get five more bread rolls 1 2 3 for five and now you can see the flower has run out the flower is the first thing to run out so it's the limiting reactant we still have one cup of water here and that's access that's what we call or excess reactant one cup of water that's left over can we run out of the limiting reactant okay so that's how you solve these kind of problems you say I want to use a maximum amount of flour how much water do I need I run out and then you say I want to try using the maximum amount of water how much flour do I need and do I run out of that here's a really important thing to mention there's a common misconception that limiting reactant is the thing that you have the least of but that's not true right look at the example that we just did we had more flower than water but flower was the limiting reactant and that's because limiting reactant isn't the thing you have the least it's the thing that runs out first so even though we had more flour than water when we're following this recipe the flower gets used up before the water does so that's why the flower is a limiting reactant even though we have more of it so keep that in mind anyway now that we work through this problem let's finally look and an example that uses a real chemical equation here's a chemical equation and we're going to use it to answer question what is the greatest amount of NH3 in moles that can be made with 3.2 moles of N2 + 5.4 pause an H2 which is the limiting reactant which reactant is in excess and how many moles of that are left over okay sell these numbers might seem a little scary and this might seem kind of confusing because it's a chemical equation but the way we're going to solve problem is almost exactly the same as how we saw the last example with a flour and water in order to find the greatest amount NH3 that we can make we first got to figure out which of these two things is the limiting reactant okay and we're going to do it exactly the play we did the previous example Ray to take the maximum amount of N2 here we're going to see how much H2 would be needed to use all of it and then we'll flip will take the maximum amount of H2 and we'll see how much into we'd need to use all that then we'll ask which do we run out of first and that's the limiting reactant okay so we'll start with the end to I've got 3.2 moles and to not really solve this stuff by Serta multiplying the recipe by number doubling it or tripling it that's a good thing to know how to do but I want to show you another strategy here okay I want to use some conversion factors to do that we look at the coefficients these numbers before each one of the compounds moles of each come together okay there's nothing in front of the end too so I want to add what all right if there's nothing in front of it it just means that it's a one okay so you can kind of see that here the equation says we have 1 mol of N2 hero 3.2 moles of N2 so it's kind of like we're multiplying this by 3.2 to figure out how much each I'm going to use a conversion factor that shows the relationship between N2 or moles of N2 and moles of a okay so I start off with 3.2 moles of N2 and then I'm going to multiply this by conversion factor that 3 moles of H2 over 1 mol of N2 that's going to tell us how much H2 we need okay so moles and two cancels moles and who cancels and we end up with  9.6 moles of H2 okay so this is how much H2 we need can use all of this M2 and again this is kind of like where multiplying by 3.2 or I should say it is like we're multiplying by 3.2 it's just another way to get this okay so this is how much H2 we need for this much into now let's do the opposite okay we have 5.4 Mol H2 I want to figure out how much into are we going to need to use all of that okay so in this case we're going to start out with here it is 5.4 moles of H2 I want to get rid of H2 so they're put H2 on the bottom here three moles of H 1 mol of N2 moles H2 cancels out and we do this math 5.4 x 1/3 is going to give 1.8 moles of N2 here it is  can you can see that we're going from one 1.8 so even though we use conversion factors to solve this it's kind of like we are multiplying both of these by 1.8 okay so now have all the information that we need to figure out which one of these is a living room is it end to or is it H2 okay well if we start with 3.2 moles of N2 weenie 9.6 moles of H2 but we don't have that much we only have 5.4 moles of H2 okay the H2 is going to be the first thing to run out on the other hand if we start with 5.4 moles of H2 we only need one 8 moles of N2 we have plenty of them so just goes to show that we've got plenty of into an H2 is going to be the first thing to run out so that means that two is are limiting reactant so now we want to know what's the greatest amount of NH3 that we can make while we have to start with this amount of H2 because that's our limiting reactant so I'm going to multiply this by I need a multiplication signs there it is multiply this by 2 moles and H 3/3 moles of H2  moles H2 moles H2 cancels out and I 3.6 moles of NH3 that's the maximum that I can make okay in there it is 3.6 moles of NH3 it's kind of like we multiply this by 1.8 so this number right there is the maximum that we can make without sing out of any of the reactants finally let's talk excess reactant if H2 is the limiting reactant excess N2 the bat is Art excess reactant okay let's figure out how much of it is leftover okay when we start with 3.2 3.2 moles of N2 and then we subtract how much we use we only use 1.8 moles of hissim - 1.8 moles and that's going to give us 1.4 moles into for our access that's how much we have left over after the H2 runs out and we can't do this reaction anymore okay add that is how you solve limiting reactant problems first you figure out which of your reactants is the limiting reactant and using the maximum amount of the limiting reactant you figure out how much of the product you can make for the excess reactant you're going to calculate how much do you have left over after the reaction stops because you've used up all the limiting reactant now living reactant can be one of the most difficult UPS in chemistry so I really urge you to watch some other videos do a lot of practice problems so this becomes very

Uploaded by Laura Houston.

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Stoichiometry: Introduction to limiting reactants

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